1. Field of the Invention
The present invention relates to a photoelectric type fire detector in a fire alarm system, or more particularly, to a self-contained self-test.
2. Description of the Related Art
A photoelectric type fire detector includes a light emitting element and a light receiving element both lying in a dark chamber. Light emanating from the light emitting element is scattered with smoke. The scattered light is detected by the light receiving element. The detected quantity of light is amplified by an amplifier. The level of an output signal of the amplifier is analyzed to determine a smoke density. Thus, fire monitoring is effected. The photoelectric type fire detector not only performs fire monitoring, but also performs what is referred to as stationary value monitoring. For stationary value monitoring, a stationary value (which is output by the amplifier in a non-fire state) is detected in the photoelectric type fire detector, and then a trouble in the photoelectric type fire detector is identified using the detected stationary value.
The stationary value is much smaller than the output levels of the amplifier resulting from the occurrence of a fire. When the stationary value is used as it is, it is hard to determine whether the photoelectric type fire detector is abnormal.
A prior art for allowing a photoelectric type fire detector to detect an own trouble is described in Japanese Examined Patent Publication No. 64-4239. The prior art has a light emitting element, a light receiving element for receiving light from the light emitting element, and an upper limit comparator and a lower limit comparator for comparing an output signal of the light receiving element with an upper limit and a lower limit respectively. A fire receiver is used to remotely control the comparators in the photoelectric type fire detector.
In the above prior art, the photoelectric type fire detector itself cannot detect its own trouble without controlling the comparators in the photoelectric type fire detector from the fire receiver. This results in a heavy work load on the fire receiver.